RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
10. Appendix .................................................... 1811. Authors' Present Addresses .................................. 191. Introduction
This document defines the various criteria to be used when designing
an Autonomous System Border Router (ASBR) that will run BGP4
[RFC1654] or IDRP for IP [IDRP] with other ASBRs external to the AS,
and OSPF [RFC1583] as its IGP.
All future references of BGP in this document will refer to BGP
version 4, as defined in [RFC1654]. All reference to IDRP are
references to the Inter-Domain Routing Protocol (ISO 10747) which has
been defined by the IDRP for IP document [IDRP] for use in Autonomous
Systems.
This document defines how the following fields in OSPF and attributes
in BGP/IDRP are to be set when interfacing between BGP/IDRP and OSPF
at an ASBR:
IDRP came out of the same work as BGP, and may be consider a follow
on to BGP-3 and BGP-4. Most fields defined in the interaction
between BGP and IDRP are named the same. Where different, the IDRP
fields are shown separately.
BGP/IDRP MULTI_EXIT_DISC
BGP ORIGIN and AS_PATH/AS_SET vs. OSPF tag
IDRP EXT_INFO and RD_PATH/RD_SET
BGP/IDRP NEXT_HOP vs. OSPF Forwarding Address
BGP/IDRP LOCAL_PREF vs. OSPF cost and type
IDRP contains RD_PATH and RD_SET fields which serves the same purpose
as AS_PATH and AS_SET fields for IDRP for IP. In this document, we
will use the terms PATH and SET to refer to the BGP AS_PATH and
AS_SET, or the IDRP RD_PATH and RD_SET fields respectively, depending
on the context of the protocol being used.
Both IDRP and BGP provide a mechanism to indicate whether the routing
information was originated via an IGP, or some other means. In IDRP,
if route information is originated by means other than an IGP, then
the EXT_INFO attribute is present. Likewise, in BGP, if a route
information is originated by means other than an IGP, then the ORIGIN
attribute is set to <EGP> or <INCOMPLETE>. For the purpose of this
document, we need to distinguish between the two cases:
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RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
(a) Route information was originated via an IGP,
(b) Route information was originated by some other means.
The former case is realized in IDRP by not including the EXT_INFO
attribute, and in BGP by setting the BGP ORIGIN=<IGP>; The latter
case is realized by including the EXT_INFO attribute in IDRP, and by
setting the BGP ORIGIN=<EGP>. For the rest of the document, we will
use the BGP ORIGIN=<IGP> to refer to the former scenario, and BGP
ORIGIN=<EGP> to refer to the latter.
One other difference between IDRP and BGP remains. IDRP for IP
identifies an autonomous system by an identifier of variable length
that is syntactically identical to an NSAP address prefix, and
explicitly embeds the autonomous system number [IDRP]. BGP
identifies an autonomous system just by an autonomous system number.
Since there is a one-to-one mapping between how an autonomous system
is identified in IDRP and in BGP, in this document, we shall identify
an autonomous system by its autonomous system number.
For a more general treatise on routing and route exchange problems,
please refer to [ROUTE-LEAKING] and [NEXT-HOP] by Philip Almquist.
This document uses the two terms "Autonomous System" and "Routing
Domain". The definitions for the two are below:
The term Autonomous System is the same as is used in the BGPRFC
[RFC1267], given below:
"The use of the term Autonomous System here stresses the fact
that, even when multiple IGPs and metrics are used, the
administration of an AS appears to other ASs to have a single
coherent interior routing plan and presents a consistent picture
of what destinations are reachable through it. From the
standpoint of exterior routing, an AS can be viewed as monolithic:
reachability to destinations directly connected to the AS must be
equivalent from all border gateways of the AS."
The term Routing Domain was first used in [ROUTE-LEAKING] and is
given below:
"A Routing Domain is a collection of routers which coordinate
their routing knowledge using a single [instance of a] routing
protocol."
By definition, a Routing Domain forms a single Autonomous System, but
an Autonomous System may be composed of a collection of Routing
Domains.
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RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
BGP, IDRP and OSPF have the concept of a set of reachable
destinations. This set is called NLRI or Network Layer Reachability
Information. The set can be represented either as an IP address
prefix, or an address, mask pair. Note that if the mask is
contiguous in the latter, then the two representations are
equivalent. In this document, we use the term "address/mask pair" in
the context of OSPF, and "destination" or "set of reachable
destinations" in the context of BGP or IDRP.
This document follows the conventions embodied in the Host
Requirements RFCs [RFC1122, RFC1123], when using the terms "MUST",
"SHOULD," and "MAY" for the various requirements.
A minimal implementation of BGP/IDRP OSPF exchange MUST not advertise
a route containing a set of reachable destinations when none of the
destinations in the address/mask pair is reachable via OSPF (section
2.1, bullet 3), MUST merge the PATH into a SET when multiple exit
points exist within the same autonomous system for the same external
destination (section 3), MUST set the OSPF tag accurately (section
4). This subset is chosen so as to cause minimal havoc to the
Internet at large. It is strongly recommended that implementors
implement more than a minimalistic specification.
2. Reachability Information Exchange
This section discusses the constraints that must be met to exchange
the set of reachable destinations between an external BGP/IDRP peer
from another AS and internal OSPF address/mask pairs.
2.1. Exporting OSPF information into BGP1. The administrator MUST be able to selectively export
address/mask pairs into BGP/IDRP via an appropriate filter
mechanism.
This filter mechanism MUST support such control with the
granularity of an address/mask pair.
This filter mechanism will be the primary method of
aggregation of OSPF internal and type 1 and type 2 external
routes within the AS into BGP/IDRP.
Additionally, the administrator MUST be able to filter based
on the OSPF tag and the various sub-fields of the OSPF tag.
The settings of the tag and the sub-fields are defined in
section 4 in more detail.
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RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
o The default MUST be to export no routes from OSPF into
BGP/IDRP. A single configuration parameter MUST permit
all OSPF inter-area and intra-area address/mask pairs to
be exported into BGP/IDRP.
OSPF external address/mask pairs of type 1 and type 2
MUST never be exported into BGP/IDRP unless they are
explicitly configured.
2. An address/mask pair having a non-contiguous mask MUST not be
exported to BGP/IDRP.
3. When configured to export an address/mask pair from OSPF into
BGP/IDRP, the ASBR MAY advertise the route containing the set
of reachable destinations via BGP/IDRP as soon as at least
one of the destinations in the address/mask pair is
determined to be reachable via OSPF; it MUST stop advertising
the route containing the set of reachable destinations when
none of the destinations in the address/mask pair is
reachable via OSPF.
4. The network administrator MUST be able to statically
configure the BGP/IDRP attribute MULTI_EXIT_DISC attribute to
be used for any route.
o The default MUST be to omit the MULTI_EXIT_DISC in the
route advertised via BGP/IDRP.
5. An implementation of BGP/IDRP and OSPF on an ASBR MUST have a
mechanism to set up a minimum amount of time that must elapse
between the learning of a new address/mask pair via OSPF and
subsequent advertisement of the address/mask pair via
BGP/IDRP to the external neighbours.
o The default value for this setting MUST be 0, indicating
that the address/mask pair is to be advertised to the
neighbour BGP/IDRP peers instantly.
Note that BGP and IDRP mandate a mechanism to dampen the
inbound advertisements from adjacent neighbours. See
the variable MinRouteAdvertisementInterval in section
9.2.3.1, [RFC1654] or in section 7.17.3.1, [IS10747].
6. LOCAL_PREF is not used when exporting OSPF information into
BGP/IDRP, as it is not applicable.
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RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
2.2. Importing BGP/IDRP information into OSPF1. BGP/IDRP implementations SHOULD allow an AS to control
announcements of BGP/IDRP learned set of reachable
destinations into OSPF. Implementations SHOULD support such
control with the granularity of a single destination.
Implementations SHOULD also support such control with the
granularity of an autonomous system, where the autonomous
system may be either the autonomous system that originated
the information or the autonomous system that advertised the
information to the local system (adjacent autonomous system).
o The default MUST be to import nothing from BGP/IDRP into
OSPF. Administrators must configure every destination
they wish to import.
A configuration parameter MAY allow an administrator to
configure an ASBR to import all the set of reachable
destinations from BGP/IDRP into the OSPF routing domain.
2. The administrator MUST be able to configure the OSPF cost and
the OSPF metric type of every destination imported into OSPF.
The OSPF metric type MUST default to type 2. If the
LOCAL_PREF value is used to construct the OSPF cost, one must
be extremely careful with such a conversion. In OSPF the
lower cost is preferred, while in BGP/IDRP the higher value
of the LOCAL_PREF is preferred. In addition, the OSPF cost
ranges between 1 and 2^24, while the LOCAL_PREF value ranges
between 0 and 2^32. Note that if ASBRs within a domain are
configured to correlate BGP/IDRP and OSPF information (as
described in Section 3), then the route selection by the
ASBRs is determined solely by the OSPF cost, and the value
carried by the LOCAL_PREF attribute has no impact on the
route selection.
3. Information learned via BGP/IDRP from peers within the same
AS MUST not be imported into OSPF.
4. The ASBR MUST never generate a default destination into the
OSPF routing domain unless explicitly configured to do so.
A default destination is a set of all possible destinations.
By convention, it is represented as a prefix of 0 length or a
mask of all zeroes.
A possible criterion for generating default into an IGP is to
allow the administrator to specify a set of (set of reachable
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RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
destinations, PATH, default cost, default type) tuples. If
the ASBR learns of at least one of the destinations in the
set of reachable destinations, with the corresponding PATH,
then it generates a default destination into the OSPF routing
domain, with the appropriate cost and type. The lowest cost
route will then be injected into the OSPF routing domain.
This is the recommended method for originating default
destinations in the OSPF routing domain.
5. Note that [RFC1247] requires the network number to be used as
the Link State ID. This will produce a conflict if the ASBR
tries to import two destinations, differing only in their
prefix length. This problem is fixed in [RFC1583], which
obsoletes [RFC1247].
An implementation conforming to the older [RFC1247] MUST, in
this case, drop the more specific route, i.e. the route
corresponding to the longer prefix in the reachability
information.
6. MULTI_EXIT_DISC is not used to import BGP/IDRP information
into OSPF, as it is not applicable.
3. BGP/IDRP Identifier and OSPF router ID
The BGP/IDRP identifier MUST be the same as the OSPF router id at all
times that the router is up.
Note that [RFC1654] requires that the BGP identifier be an address
assigned to the BGP speaker.
In the case of IDRP, the IDRP protocol does not explicitly carry the
identity of the IDRP speaker. An implicit notion of the identity of
the IDRP speaker can be obtained by examining the source address in
the IP packets carrying the IDRP information. Therefore, all IDRP
speakers participating in the OSPF protocol MUST bind the IDRP
identifier to be the address of the OSPF router id.
This characteristic makes it convenient for the network administrator
looking at an ASBR to correlate different BGP/IDRP and OSPF
information based on the identifier. There is another more important
reason for this characteristic.
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RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
Consider the scenario in which 3 ASBRs, RT1, RT2, and RT3, belong to
the same autonomous system.
+-----+
| RT3 |
+-----+
|
Autonomous System running OSPF
/ \
+-----+ +-----+
| RT1 | | RT2 |
+-----+ +-----+
Both RT1 and RT2 can reach an external destination X and import this
information into the OSPF routing domain. RT3 is advertising this
information about destination X to other external BGP/IDRP speakers.
The following rule specifies how RT3 can generate the correct
advertisement.
RT3 MUST determine which ASBR(s) it is using to reach destination X
by matching the OSPF router ID for its route to destination with the
BGP identifier of the ASBR(s), or the IP source address of the IDRP
protocol packet from the ASBR(s).
o If RT3 has equal cost routes to X through RT1 and RT2, then,
RT3 MUST merge the PATH through RT1 and RT2 into a SET.
o Otherwise, RT3 MAY merge the PATH through RT1 and RT2.
It MAY then generate the corresponding network layer reachability
information for further advertisement to external BGP/IDRP peers.
4. Setting OSPF tags, ORIGIN and PATH attributes
The OSPF external route tag is a "32-bit field attached to each
external route . . . It may be used to communicate information
between AS boundary routers; the precise nature of such information
is outside the scope of [the] specification" [RFC1583].
We use the external route tag field in OSPF to intelligently set the
ORIGIN and PATH attributes in BGP/IDRP. These attributes are well-
known, mandatory attributes in BGP/IDRP. The exact mechanism for
setting the tags is defined in sections 4.2 and 4.3. Every
combination of tag bits is described in two parts:
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RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
import This describes when an ASBR imports an AS external LSA into
the OSPF domain with the given tag setting.
export This indicates how the BGP/IDRP path attribues should be
formatted when an ASBR, having a given type 1 or type 2
OSPF external route in its routing table, decides to export
according to the considerations in section 2.1.
The tag is broken up into sub-fields shown below. The various
sub-fields specify the characteristics of the set of reachable
destinations imported into the OSPF routing domain.
The high bit of the OSPF tag is known as the "Automatic" bit.
Setting this bit indicates that the tag has been generated
automatically by an ASBR.
When the network administrator configures the tag, this bit MUST be
0. This setting is the default tag setting, and is described in
section 4.2.
When the tag is automatically generated, this bit is set to 1. The
other bits are defined below:
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|c|p l| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
c 1 bit of Completeness information, set when the ORIGIN of the
route is either <EGP> or <IGP>.
pl 2 bits of PathLength information; this field is set depending
on the length of the PATH that the protocol could have carried
when importing the reachability information into the OSPF
routing domain.
ArbitraryTag
12 bits of tag information, defaults to 0 but can be
configured to anything else.
AutonomousSystem (or "AS")
16 bits, indicating the AS number corresponding to the set of
reachable destinations, 0 if the set of reachable destinations
is to be considered as part of the local AS.
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RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
local_AS: The AS number of the local OSPF routing domain.
next_hop_AS: The AS number of an external BGP peer.
4.1. Configuration parameters for setting the OSPF tag
o There MUST be a mechanism to enable automatic generation of
the tag characteristic bits.
o Configuration of an ASBR running OSPF MUST include the
capability to associate a tag value, for the ArbitraryTag, or
LocalInfo sub-field of the OSPF tag, with each instance of a
routing domain.
o Configuration of an ASBR running OSPF MUST include the
capability to associate an AS number with each instance of a
routing domain.
Associating an AS number with an instance of an IGP is
equivalent to flagging those set of reachable destinations
imported from the IGP to be external destinations outside the
local autonomous system.
4.2. Manually configured tags
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| LocalInfo |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
import This tag setting corresponds to the administrator manually
setting the OSPF tag bits.
export The route SHOULD be exported into BGP with the attributes
ORIGIN=<EGP>, PATH=<local_AS>.
Nothing MUST inferred about the characteristics of the set of
reachable destinations corresponding to this tag setting.
For backward compatibility with existing implementations of OSPF
currently deployed in the field, this MUST be the default setting
for importing destinations into the OSPF routing domain. There
MUST be a mechanism to enable automatic tag generation for
imported destinations.
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RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
import These are reachable destinations imported from routing
protocols with either complete path information, or are
known to be complete through means other than that
carried by the routing protocol. The routing protocol
also has additional information about the next hop AS
or RD, the destination was learned from.
This setting SHOULD be used when the administrator
explicitly associates an AS number with an instance of
an IGP. This setting MAY also be used when importing
reachable destinations from BGP/IDRP whose ORIGIN=<IGP>
and PATH=<next_hop_AS>; if the BGP/IDRP learned route
has no other transitive attributes, then its
propagation via BGP/IDRP to other ASBRs internal to the
autonomous system MAY be suppressed.
export OSPF routes with this tag setting SHOULD be exported
with the BGP/IDRP attributes, ORIGIN=<IGP>,
PATH=<local_AS, next_hop_AS>.
4.3.6. Tag = <Automatic = 1, Complete = 1, PathLength = 10>
0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|1|1|0| ArbitraryTag | AutonomousSystem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
import These are reachable destinations imported from routing
protocols with complete path information and carry the
AS path information as part of the routing information.
These destinations MUST not be exported into BGP/IDRP
because these are destinations that are already
imported from BGP/IDRP into the OSPF RD. Hence, it is
assumed that the BGP/IDRP speaker will convey these
routes to other BGP/IDRP speakers within the same
autonomous system via BGP/IDRP. An ASBR learning of
such a destination MUST wait for the BGP update from
its internal neighbours before advertising it to
external BGP/IDRP peers.
export These routes MUST not be exported into BGP/IDRP.
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RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
- Exporting a reachable destination into BGP:
When exporting set of reachable destinations internal to the
OSPF routing domain from OSPF to BGP/IDRP, if all the
destinations in the set of reachable destinations are through
RT4, then RT3 MAY fill the NEXT_HOP attribute for the set of
reachable destinations with the address of RT4. This is to
avoid requiring packets to take an extra hop through RT3 when
traversing the AS boundary. This is similar to the concept of
indirect neighbour support in EGP [RFC888, RFC827].
6. Changes from the BGP 3 - OSPF interactions document
o The use of the term "route" has attained a more complicated
structure in BGP 4. This document follows the constraint in
the manner shown below:
- The term "set of reachable destinations" is called a NLRI
in [RFC1654].
- The term "route" in the BGP context refers to a set of
reachable destinations, and the associated attributes for
the set.
- The term "route" in the OSPF context refers to the set of
reachable destinations, and the cost and the type to
reach destinations. This is to keep the definitions
consistent in the document.
o The notion of exchanging reachability information between BGP
4 and OSPF has been updated to handle variable length net mask
information.
o The previous term INTER_AS_METRIC in BGP 3 has now been
changed to MULTI_EXIT_DISC.
o The default metric to be used for importing BGP information
into the OSPF RD is now the LOCAL_PREF attribute, instead of a
constant value.
o Section 3 which requires an ASBR to match the OSPF tag
corresponding to a route to the BGP Identifier, can cause
potential loops if the AS has equal cost multipath routing
amongst the ASBRs. This scenario is outlined in the Appendix
below. This is fixed in BGP4 by requiring the ASBR seeing
equal cost multi-path routes to merge the PATHs through the
various ASBRs into appropriate SETs.
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RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994
o BGP 4 requires that the BGP identifier be an address assigned
to the BGP speaker. This is dealt with in section 3.
o Section 5 on setting NEXT_HOP attributes and Forwarding
Address field has been updated to account for variable length
net information.
o This section, 6, has been added.
7. Security Considerations
Security issues are not discussed in this memo.
8.Acknowledgements
We would like to thank Jeff Honig (Cornell University), John Moy
(Cascade Communications Corp.), Tony Li (cisco Systems), Rob Coltun
(Consultant), Dennis Ferguson (ANS, Inc.), Phil Almquist
(Consultant), Scott Bradner (Harvard University), and Joel Halpern
(Newbridge Networks Inc.) for their help and suggestions in writing
this document. Cengiz Aleattinoglu (USC/ISI) and Steve Hotz
(USC/ISI) provided fresh insights into the packet looping problem
described in the appendix.
We would also like to thank the countless number of people from the
OSPF and BGP working groups who have offered numerous suggestions and
comments on the different stages of this document.
Thanks also to Bob Braden (ISI), whose suggestions on the earlier
BGP-OSPF document, [RFC1403] were useful even for this one, and have
been carried through.
We would also like to thank OARnet, where one of the authors did most
of his work on this document, before moving to USC to resurrect his
PhD.
9. Bibliography
[RFC827] Rosen, E., "Exterior Gateway Protocol (EGP)", RFC 827,
BBN, October 1982.
[RFC888] Seamonson, L., and E. Rosen, "`STUB' Exterior Gateway
Protocol", RFC 888, BBN, January 1984.
[RFC1058] Hedrick, C, "Routing Information Protocol", RFC 1058,
Rutgers University, June 1988.
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